The subject matter of the present invention pertains to means for minimizing the effect of undesirable excusions or aberrations within the output of a digital-to-analog converter.
As is known to the art, digital-to-analog converters (DAC's) are usually formed in sections, with each section being controlled by a preselected subset of the bits within an input digital signal. For example, a 12-bit DAC might be divided into four sections, each of which is controlled by three bits of the input signal. As the input changes, the output of each affected section changes; the output of the DAC being the composite sum of the outputs from each section. Ideally, the output of the DAC is a series of analog step functions, the magnitudes and directions of which are a function of the changes in the digital input.
As is also known to the art, significant excursions (spikes, glitches) occur in the output signal in response to certain changes in the input. These excursions--the result of delays inherent in the DAC circuitry, out-of-synch switching of internal current sources, and other known causes--are especially prevalent at points of major transition; a major transition being a change in the input signal causing the DAC to switch from one internal section to another. In the example cited above, assuming a linear relationship between the input bits and the DAC sections, a major transition would occur when the input signal changes from 0111.sub.2 to 1000.sub.2 (first section deactivated, second section activated) or in the opposite direction from 001000.sub.2 to 000111.sub.2 (second section deactivated, first section activated) or between any two values causing a particular DAC section to be deactivated at the same time another section is activated. In the more general case, where each input bit controls a single one-bit section, a worst-case major tansition occurs when one bit is changed in one direction at the same time that all remaining bits are changed in the other direction; for example, when the input switches from 01111111.sub.2 to 10000000.sub.2. The effect is less when less bits are switched. In either case, although the excursions thus produced are of fairly short duration--on the order of several hundred nanoseconds, or less--they may still extend over several cycles of a faster changing input. In a graphics display environment wherein DAC's are used, for example, to drive callidgraphically an electron beam in a cathode-ray tube, such excursions are observable as nonlinearities in beam movement and non-uniformities in beam intensity.
Known attempts to remove or minimize undesirable excursions in DAC outputs include the use of both linear filtering and sample-and-hold techniques. Although linear filtering reduces the amplitude of the excursion--by integration over a longer period of time--it does not reduce the excursion energy. Sample-and-hold techniques, while satisfactory at low input rates, are difficult to realize at very high rates and often produce their own component of noise and signal excursions.
Another known attempt to solve the problem of unacceptable excursions in a DAC output is the non-linear filter disclosed in Rieger et al. U.S. Pat. No. 4,163,948 assigned to Tektronix, Inc., the assignee of the present invention. Using slew-rate techniques, the Rieger et al. filter removes all of some excursions and parts of others; however, it is less capable of handling adequately excursions extending over more than one change of the input signal.